Different kinds of substrates such as a sapphire substrate, a silicon substrate or the like are used as a substrate to grow a nitride-compound semiconductor.
Thermal expansion coefficient of the nitride-compound semiconductor is different from that of the different kind of each substrate. Therefore, warpage is generated in the stacked nitride-compound semiconductor structure which is constituted with nitride-compound semiconductors on the different kind of the substrate.
Conventionally, a stacked nitride-compound semiconductor structure mentioned below has been well known. Nitride-compound semiconductor layers with an active layer and another nitride-compound semiconductor layer are provided on a surface and a back surface of the substrate, respectively, in the stacked nitride-compound semiconductor structure.
Such the stacked nitride-compound semiconductor structure is fabricated as follows to decrease the warpage. First, the nitride-compound semiconductor layer is formed on the back surface of the substrate. Next, the nitride-compound semiconductor layers with the active layer are formed on the surface of the substrate.
In the processing steps to provide the nitride-compound semiconductor layer on the back surface of the substrate, process gas is penetrated into a side of the surface of the substrate. Further, deposition substances are attached to an outer periphery portion of the surface of the substrate. When the deposition substances remain, crystalline defects are generated in the nitride-compound semiconductor layers with the active layer so as to degrade device characteristics and device yield.
Accordingly, it is necessary to remove the deposition substances on the substrate without damage to the surface of the substrate prior to form the nitride-compound semiconductor layers with the active layer on the substrate.
However, nitride-compound semiconductor has a strong resistance to chemicals. As a result, there arises a problem that the deposition substances cannot be sufficiently removed to leave residues.
When the different kind of the substrate is composed of silicon, especially, there arises a problem that even retaining flatness of a surface of the substrate is difficult, as silicon is strongly reacted with gallium.
It is considered that the surface of the silicon substrate is polished again. However, the stacked nitride-compound semiconductor structure has warpage in this step. Therefore, the structure has difficulty even in retaining the flatness of the surface of the substrate.
Furthermore, warpage is proportional to square of a diameter. When a substrate with larger diameter is used in the process, more severe problem may be generated.